Proton Exchange Membrane (PEM) fuel cell power systems are currently not available in the open market because of two main factors, namely, (1) the lack of robust and efficient hydrogen generators that produce pure hydrogen and (2) the high cost and low efficiency of PEM fuel cell stack or stacks when run on a reformate of H2 fed to the anode side (PEM fuel cell efficiency=electrical power output/LHV (Lower Heating Value); Hydrogen generator efficiency=LHV of H2 produced/LHV of feed hydrocarbons). Typically, a reformate is composed of hydrogen and diluents such as CO2, N2, hydrocarbons and contaminants such as CO. Although these two factors appear to be independent, they are not. For the same power output, the size of a PEM fuel cell stack that runs on pure hydrogen (>99.9%) can be 30% smaller than a PEM fuel cell stack that runs on a reformate. This is because the most preferred method of operating a PEM fuel cell is to run its anode side “dead-ended”, that is, except for an occasional short time purge, all of the hydrogen that enters the anode side of each cell in the PEM fuel cell stack is utilized (dissociated into H+ ions). When a reformate is fed to the anode side of the fuel cell, the anode side has to be operated in a continuous purge mode (on account of the presence of diluents) and cannot be run “dead-ended”. Under such conditions, hydrogen utilization is on the order of only 85%, which means that at least 15% of the hydrogen that is produced by the hydrogen generator is practically “wasted” in the fuel cell. This also raises an additional problem of finding ways to utilize this “wasted” hydrogen, for example by burning this hydrogen to generate useful heat. The efficiency of the PEM fuel cell under such conditions is only about 40%. Assuming an efficiency of about 60% for the hydrogen generator (typically about 50 to 70%), the overall system efficiency works to be only about 24% (=40% x60%). It becomes obvious that at these system efficiencies, a PEM fuel cell power system is several times more expensive than an internal combustion (IC) engine (about >$3000/KW vs <about $50/KW, respectively), with no appreciable gains in efficiency. As a result of this, from a technical standpoint, there is no incentive to install PEM fuel cell power systems on a commercial scale.
On the contrary, when pure hydrogen is supplied to a PEM fuel cell, the efficiency of PEM fuel cells can be as high as 60%, resulting in an overall system efficiency of 36%. At these efficiencies, PEM fuel cell power systems become attractive, particularly when associated features such as lower pollution and lower noise are taken into consideration. The viability of PEM fuel cell systems is then reduced to issues related to cost and system reliability.
One method by which hydrogen may be produced for use in a PEM fuel cell utilizes steam reformers to provide hydrogen from a hydrocarbon fuel supply. Other methods such as ammonia cracking, auto thermal reforming, partial oxidation of hydrocarbons also provide hydrogen in the form of a reformate. In order to separate the hydrogen so produced by reformers, a hydrogen separation membrane may be employed, which is comprised of various metals/alloys. A pressure swing adsorption unit (PSA) can also be employed for hydrogen purification
Various metals/alloys are known to be permeable to hydrogen and are thus useful as separation membranes. For example, Pd metal and many of its alloys are well known for their ability to dissociate and diffuse hydrogen.
Hydrogen separation membranes can be in the form of thin coatings deposited on porous planar or tubular supports or can be solid tubular membranes (>70 microns in thickness) as sold by Johnson Matthey. The latter are used for purifying hydrogen for the electronic industry but are not suitable for hydrogen purification for fuel cell use. In the former case, one may employ the use of various supports, such as ceramics, steel and other supports/substrates onto which these membranes are disposed. It has been noted that prior art deposition processes provide resultant hydrogen separation membranes which are expensive, inefficient and do not have desirable physical stability and lifetime characteristics.